1. Field of the Invention
The present invention relates to an electric power steering system for assisting steering force with a motor.
2. Description of the Prior Art
A conventional electric power steering system determines steering assist force based on a steering torque applied to a steering shaft connected to a steering wheel and a car speed and controls the output torque of a motor connected to a steering system based on this steering assist force to assist the steering force of the steering system. To improve a steering feeling, the output torque of the motor is corrected based on the angular velocity or angular acceleration of the motor, the time differential value of steering torque or the like.
FIG. 13 is a diagram showing the constitution of software used for the steering control of a conventional electric power steering system disclosed by Japanese Laid-open Patent Application No. 8-175404, for example. In FIG. 13, reference numeral 1 denotes a motor, connected to a steering system through a speed reducer, for generating steering assist force, 2 motor angular velocity computing means for computing the back electromotive force of the motor from a motor detection current. Im and a motor application voltage Vm to obtain an estimated value of rotation angular velocity (to be referred to as “motor angular velocity” hereinafter) ωm of the motor 1 and outputting the value as a motor angular velocity estimated value ω, and 3 motor angular acceleration computing means for computing a motor angular acceleration by differentiating the above motor angular velocity estimated value ω and outputting the value as a motor angular acceleration estimated value (dω/dt). Reference numeral 4 represents coulomb friction compensation current computing means for computing a coulomb friction compensation current Ic for compensating for the coulomb friction of the steering system based on the above motor angular velocity estimated value ω, 5 viscous friction compensation current computing means for computing a viscous friction compensation current Id for compensating for the viscous friction of the steering system based on the above motor angular velocity estimated value ω, 6 inertia compensation current computing means for computing an inertia compensation current Ij for compensating for the inertia moment of the steering system based on the motor angular acceleration estimated value (dω/dt), 7 steering force assist current computing means for computing a steering force assist current Is for assisting the steering force of a driver based on the steering torque Vt of the driver, and 8 current control means for feedback controlling the drive current of the motor 1 so that the above motor detection current Im and a motor target current Isum obtained by adding together the above coulomb friction compensation current Ic, the viscous friction compensation current Id, the inertia compensation current Ij and the steering force assist current Is become equal to each other.
A description is subsequently given of the operation of the electric power steering system. The motor angular velocity computing means 2 computes an estimated value ω of motor angular velocity ωm from a motor detection current Im and a motor application voltage Vm. The motor 1 is a separately excited DC motor. When the motor 1 turns to assist the steering force of a driver by his operation of the steering wheel, a back electromotive force Ve proportional to the motor angular velocity ωm is generated in the motor 1. The relationship between the motor angular velocity ωm and the back electromotive force Ve of the motor can be expressed by the following expression (1):Ve=Ke·ωm  (1)wherein Ve is a back electromotive force of the motor (V), Ke is a constant of the back electromotive force of the motor (V·s/rad) and ωm is a motor angular velocity (rad/s).
The motor angular velocity computing means 2 computes a motor back electromotive force Ve based on the following expression (2) from a motor detection current Im and a motor application voltage Vm:Ve=Vm−Im·Ra  (2)wherein Ra is a resistance of a motor armature.
The estimated value ω of motor angular velocity ωm is computed from the expression ω=(Vm−ImRa)/Ke using the value of motor back electromotive force Ve obtained from the above expression (2) and the above expression (1). This motor angular velocity estimated value ω is output to the motor angular acceleration computing means 3, the coulomb friction compensation current computing means 4 and the viscous friction compensation current computing means 5.
The motor angular acceleration computing means 3 obtains a motor angular acceleration estimated value (dω/dt) by carrying out differential operation on the above motor angular velocity estimated value ω from the motor angular velocity computing means 2 and outputs it to the inertia compensation current computing means 6.
In the electric power steering system, since the friction of the motor 1 is transmitted to the steering system, the friction of the steering system increases, whereby the returnability of the steering wheel at a low speed may deteriorate.
The coulomb friction compensation current computing means 4 computes a coulomb friction compensation current Ic for compensating for an increase in the friction of the steering system. As shown in FIG. 14, this coulomb friction compensation current Ic is a current having a fixed value which is given when the motor angular velocity estimated value ω exceeds a predetermined value ω0 so that steering assist force is applied in the same direction as the rotation direction of the motor 1.
Since the motor 1 is connected in the electric power steering system, the inertia moment of the steering system increases. This leads to an increase in steering force when the steering wheel is turned quickly or the poor maneuverability of the steering wheel when the lane is changed at a high speed.
As shown in FIG. 15, the inertia compensation current computing means 6 gives an inertia compensation current Ij proportional to the motor angular acceleration estimated value (dω/dt), thereby improving the response of the electric power steering system.
As shown in FIG. 16, the viscous friction compensation current computing means 5 gives a viscous friction compensation current Id proportional to the motor angular velocity estimated value ω and having polarity opposite to that of the value, thereby improving the attenuation of the electric power steering system.
The steering force assist current computing means 7 gives a steering force assist current Is for assisting steering force in accordance with a car speed Vs and a steering torque Vt as shown in FIG. 17 when the driver operates the steering wheel and the steering torque Vt rises, thereby reducing the steering force of the driver. The value of the steering force assist current Is increases as the car speed Vs decreases.
The above currents (Ic, Id, Ij, Is) thus obtained are added together to obtain the target current Isum of the motor 1. The current control means 8 feedback controls the drive current of the motor 1 and drives the motor 1 so that the motor target current Isum and the motor detection current Im become equal to each other.
Friction can be modeled as shown in FIG. 18 according to “University Lecture, Automatic Control” written by Masami Ito. That is, when an object stands still, static friction proportional to the vertical resistance of the object works and when the object has a speed, viscous friction proportional to the speed of the object works based on dynamic friction called “coulomb friction”. In contrast to this, in the conventional electric power steering system, the coulomb friction and the viscous friction of the steering system are compensated while static friction is not taken into consideration. Therefore, the conventional electric power steering system has such a problem as a bad steering feeling at around the neutral location of the steering wheel that a driver feels as if the steering wheel were caught when he starts to operate the steering wheel due to the influence of this static friction (to be referred to as “on-center feeling” hereinafter).
In the electric power steering system, when the on-center feeling is heavy as described above, means of adding a current proportional to the differential value of torque to a motor current is generally employed. However, compensation with a current proportional to the differential value of torque is mainly aimed to absorb the influence of the inertia force of the motor 1 as described in Examined Japanese Patent Publication No. 3-42235 and not to compensate for the static friction of the steering system. Therefore, when an inertia compensation current Ij for compensating for the inertia of the motor 1 in the prior art is added to a current proportional to the differential value of torque, the inertia of the motor is compensated excessively, resulting in an unsteady steering feeling or the oscillation of a motor current.
It is an object of the present invention which has been made to solve the above problems to improve the steering feeling by estimating the static friction of the steering system and compensating for this static friction.